The present invention relates to control air handling units of an heating, ventilation and air conditioning system, and more particularly to regulating the amount of outdoor air that is introduced into the system in order to reduce the amount of mechanical heating and cooling required.
FIG. 1 conceptually illustrates a typical single duct air handling unit (AHU) 10 of a heating, ventilation and air conditioning (HVAC) system which controls the environment of a room 12 in a building. Air from the room is drawn into a return duct 14 from which some of the air flows through a return damper 16 to a supply duct 18. Some of the return air may be exhausted outdoor the building through an outlet damper 20 and replenished by fresh outdoor air entering through an inlet damper 22. There always is a minimum amount of fresh outdoor air entering the system for proper ventilation within the building. The dampers 16, 20, and 22 are opened and closed by actuators which are operated by a controller 24 to control the ratio of return air to fresh outdoor air. The mixture of return air and fresh outdoor air is forced by a fan 25 through a cooling coil 26 and a heating coil 28 before being fed into the room 12.
The controller 24 also operates a pair of valves 27 and 29 that regulate the flow of chilled fluid through the cooling coil 26 and the flow of heated fluid through the heating coil 28, depending upon whether the circulating air needs to be cooled or heated. These coils 26 and 28 provide xe2x80x9cmechanicalxe2x80x9d heating and cooling of the air and are referred to herein as xe2x80x9cmechanical temperature control elements.xe2x80x9d The amount of cooling or heating energy that is required to be provided by mechanical temperature control elements is referred to herein as a xe2x80x9cmechanical loadxe2x80x9d of the HVAC system.
Sensors 30 and 32 respectively measure the temperature and humidity of the outdoor air and provide signals to the controller 24. Another pair of sensors 34 and 36 respectively measure the temperature and humidity of the air in the return duct 14. Additional temperature sensors 38 and 39 are located in the outlet of the supply duct 18 and in the room 12.
The controller 24 executes a software program that implements an air side economizer function that uses outdoor air to reduce the mechanical cooling requirements for the air handling unit 10. There are three air side economizer control strategies that are in common use: temperature, enthalpy, and temperature and enthalpy. The strategies control transitions between two air circulation modes: minimum outdoor air with mechanical cooling and maximum outdoor air with mechanical cooling.
In temperature economizer control, an outdoor air temperature is compared to the return temperature or to a switch-over threshold temperature. If mechanical cooling is required and the outdoor air temperature is greater than the return air temperature or the switch-over threshold temperature, then a minimum amount of outdoor air required for ventilation (e.g. 20% of room supply air) enters air-handling unit 10. If mechanical cooling is required and the outdoor air temperature is less than the return temperature or a switch over threshold temperature, then a maximum amount of outdoor air (e.g. 100%) enters the air-handling unit 10. In this case, the outlet damper 20 and inlet damper 22 are opened fully while the return damper 16 is closed.
With enthalpy economizer control, the outdoor air enthalpy is compared with the return air enthalpy. If mechanical cooling is required and the outdoor air enthalpy is greater than the return air enthalpy, then the minimum amount of outdoor air required for ventilation enters the air-handling unit. Alternatively when mechanical cooling is required and the outdoor air enthalpy is less than the return air enthalpy, then the maximum amount of outdoor air enters the air-handling unit 10.
With the combined temperature and economizer control strategy, when mechanical cooling is required and the outdoor temperature is greater than the return temperature or the outdoor enthalpy is greater than the return enthalpy, the minimum amount of outdoor air required for ventilation is used. If mechanical cooling is required and the outdoor temperature is less than the return air temperature and the outdoor enthalpy is less than the return enthalpy, then the maximum amount of outdoor air enters the air-handling unit. The parameters of either strategy that uses enthalpy have to be adjusted to take into account geographical environmental variations.
The present invention is an alternative to these three previously used control strategies.
A novel control strategy for controlling air side economizer has been developed for an HVAC system. The first embodiment of this economizer control strategy uses a model of the airflow through the system to estimate the mechanical load of the HVAC system, such as the load on a cooling coil for example, for minimum and maximum outdoor airflow into the HVAC system. Transitions between minimum outdoor air and maximum outdoor air usage occur based on those estimated mechanical loads. The second embodiment of this economizer control strategy uses the model and a one-dimensional optimization routine to determine the fraction of outdoor air that minimizes the mechanical load on the HVAC system.
The environment of a room in a building is controlled by calculating a first load on the mechanical temperature control element based on a first flow rate of outdoor air into the room, and calculating a second load based on a second flow rate of outdoor air into the room. In the preferred embodiment of the control method the first flow rate is the maximum amount of outdoor air and the second flow rate is the minimum amount of outdoor air that is required for adequate ventilation in the room.
The first and second loads on the mechanical temperature control element are compared, and the flow rate of outdoor air into the room is regulated in response to the comparison. In the preferred operation of this control strategy, the first flow rate is used when the first load is less than the second load; and outdoor air flows into the room 12 of the building at the second when the second load is less than the first load.
Another embodiment of the present invention involves deriving a fractional flow rate of outdoor air which is between the first and second flow rates. For example, a model of the airflow through the HVAC system is used to determine the optimum fractional flow rate. In this case, a calculation is made of a third load on a mechanical temperature control element based on fractional flow rate of outdoor air being introduced into the room. The third load is used along with the first and second loads to determine the amount of outdoor air to be introduced into the room.
In this embodiment, the first amount of outdoor air is introduced into the room when the second load is greater than the first load and the third load is greater than the first load. The second amount of air is introduced into the room when the first load is greater than the second load, and the third load is greater than the second load. Finally, outdoor air is introduced into the room at the third flow rate when the second load is greater than the third load and the first load is greater than the third load.